Mutants blocked or altered at different steps in the synthesis, assembly or secretion of Ig by mouse myeloma cells have been isolated and are being characterized. Mutants have been isolated which synthesize heavy chains of altered molecular weight; these include mutants synthesizing chains with deletions in CH or in the Fc and mutants synthesizing heavy chains 1.5 and 2 times the molecular weight of wild-type chains. Mutants have been isolated which synthesize altered heavy chains which are assembled into HL half molecules instead of the complete H2L2 molecule. A mutant with a decreased level of heavy chain mRNA has been characterized. The gene encoding the heavy chain has been cloned into lambda GT-10 and has been found to have an altered 3' terminus, most probably the result of a deletion. This result suggests that sequences 3' to the CH3 exon of the heavy chain exert regulating influences altering the steady state level of cytoplasmic mRNA.